Rotary drill bits are commonly used for drilling bore holes or wells in earth formations. One type of rotary drill bit is the roller cone bit (often referred to as a “rock” bit), which typically includes a plurality of conical cutting elements secured to legs dependent from the bit body. All bits have a body with a threaded upper end for connection to a drill string. The body has three depending legs each having a bearing pin. A rotatable cone is mounted on each of the bearing pins.
One type of bit has cones that have cemented carbide inserts or compacts press-fitted into mating holes formed in the exterior of the cone. The inserts protrude past the shell for engaging and disintegrating the earth formation. The inserts are formed by compacting a mixture of tungsten carbide particles and a metal binder within a die, then heating the pressed product to sinter it. The cone shells or bodies are formed of steel, thus the carbide inserts are much more resistant to abrasive wear than the shell of the cone. In drilling applications involving extended periods of operation, or a high content of abrasive particles in the formation and drilling fluid, extensive erosion and abrasion of the cone may occur, causing a loss of inserts.
Another type of cone has teeth that are milled or machined directly into the exterior surface of the steel cone. After machining the teeth, hardfacing is applied to the teeth, gage, and other surfaces of the cone to resist wear. The hardfacing typically comprises tungsten carbide granules or pellets embedded within a ferrous based matrix. A variety of different types of hardfacing particles are employed, including cemented tungsten carbide, cast tungsten carbide, macrocrystalline tungsten carbide and mixtures thereof. Typically, the hardfacing is applied manually using an oxy-acetylene torch. During application, a technician melts a steel tube containing the hardfacing particles with the flame and deposits the material on the selected portions of the cones.
Hardfacing applications are labor intensive, not well controlled or repeatable and also may inhibit the cutting structure because of the inherent bluntness of the resulting hardfaced teeth. Some grinding of the hardfacing to desired shapes may be performed. U.S. Pat. No. 6,766,870, assigned to the assignee of the present application and incorporated herein by this reference, discloses and illustrates a method of shaping hardfaced teeth through a secondary machining operation. However, sharpening the hardfaced teeth by grinding adds another relatively difficult and expensive step in the manufacturing process. Also, the portions of the cone shell that are not hardfaced may erode extensively in abrasive drilling conditions, causing a loss of teeth or the entire cone.
Another type of drill bit is a fixed-cutter bit, which does not have rotatable cones. Instead, a plurality of polycrystalline diamond cutting elements is secured to the cutting surface of the bit. In one type, the fixed-cutter bit has a bit crown formed of a particle-matrix composite material and joined to a steel shank. The shank has a threaded upper end for connection to the drill string. The particle-matrix bit crown is typically formed by placing hard particulate material, such as tungsten carbide, titanium carbide or tantalum carbide, in a cavity of a rigid mold defining the bit topography along with an alloy matrix material, such as a copper alloy. The mold, typically constructed of graphite with insertions of resin coated casting sand components, graphite or ceramic displacements, molding clay, or other geometry defining materials, is then placed in a furnace to melt the copper alloy and infiltrate and bond the tungsten carbide particles together. A steel blank may be embedded in the mold along with the tungsten carbide particles prior to applying heat. After the heat application and completion of the matrix infiltration, the blank is machined into a configuration to allow the attachment of a threaded shank. Alternatively, the bit crown could be formed separately and subsequently bonded to a threaded steel shank.
Since the particle-matrix bit crown cannot be readily machined because of its hardness after the casting process, the cavity of the mold must be formed with the net desired shape and size for the bit. The mold is intricate and requires extensive machining and hand finishing. The mold must usually be broken subsequent to the infiltration cycle to remove the finished bit crown and used only once, making particle-matrix bits costly.
Particle-matrix material used to form fixed cutter bit crowns differs from the cemented tungsten carbide used for the press-fit inserts or cutting elements of rotating cone bits in several ways. The material of a particle-matrix crown is normally of lower strength than the material of cemented tungsten carbide cutting elements. Cemented tungsten carbide material typically used for cutting elements normally has higher compressive, tensile and bending strengths than the material of a particle-matrix bit crown. The hard particles of the particle-matrix material are typically larger than the hard particles of liquid-phase sintered material, being typically at least 20-25 microns while the tungsten particles for a cemented tungsten carbide cutting element are typically less than 20 microns. The matrix of a particle-matrix bit crown typically comprises a copper-based alloy, while the binder of a cemented tungsten carbide cutting element is formed of cobalt, nickel, iron or alloys of them. The amount of binder in a particle-matrix bit crown is about 40 to 70% by volume, while the amount of binder in a cemented tungsten carbide cutting element is about 6 to 16% by weight.
The method of forming a particle-matrix bit crown differs greatly from the method of forming cemented tungsten carbide cutting elements. A principal difference is that a particle-matrix bit crown does not undergo the application of high pressure while in a mold. Rather the tungsten carbide powder is poured in the refractory mold, which has previously been configured to define the desired topography. It is during the furnace infiltration cycle that the copper alloy matrix melts and flows between the hard particles and bonds them together. Particle-matrix bit crown bits are processed in a furnace at lower temperatures and without a controlled atmosphere. The temperature used to form a particle-matrix bit crown is typically about 1180-1200 degrees C.
A cemented tungsten carbide cutting element, by contrast, is shaped by the use of high pressure to compact the hard metal particles and metal binder prior to sintering. Sintering of cemented tungsten carbide with other than lower melting temperature binders, such as copper based alloys, requires a vacuum or controlled atmosphere furnace. In the case of cemented carbides, the binder alloy is mixed and dispersed in the carbide aggregate prior to the initial pressing to shape of the component. During the furnace sintering cycle, the admixed binder particles melt and form a continuous phase that surrounds the hard aggregate particles. There is no flow of binder material from an external source or reservoir, as in the case of the infiltration of a matrix bit crown. The temperature for sintering a tungsten carbide cutting element is about 1320-1370 degrees C. High temperature processing in an oxygen containing atmosphere at these temperatures is not possible because of the oxidation that would occur to these materials at the processing temperature. The sintering step in cemented carbide results in significant shrinkage because the porosity in the pressed particulate component is eliminated as the binder material melts and the resulting surface tension of the molten binder pulls the particles together. In the case of the particle matrix bit crown, the interstitial volumes are filled with molten metal binder that is supplied from an external reservoir without significant compaction of the particle bed. Volumetric shrinkage values in the cemented carbide typically range from 20 to 50 percent, while no significant shrinkage occurs during the heating step of a particle-matrix bit crown.